Use no-op snapshot store in restorers (2/2).

When the revamped UI flag is off, the undo system doesn't start a
session, which means it doesn't hand a "store" to its restorers. By
having all restorers start off with a safe-to-use no-op store, we
circumvent a lot of crashes.

Fix: 272157475
Fix: 271967027
Test: manually verified that the crashes no longer occur with the flag
on or off and that the undo system (reset button) still works.

Change-Id: I0a6e709d569ace7b2e8468e3f4810ae4f67a3b7d